The present invention relates generally to thermal cutting and, more particularly, to the true linear thermal severing of relatively thick glass plates or sheets.
The art of thermally severing thick glass sheets or plates has become increasingly popular in recent years as an alternative to the arduous and more expensive cutting procedures conventionally employed. One persistent problem encountered in thermally severing thick glass sheets or plates is the inability to repeatedly form accurate straight cuts. Often, the fracture or cut tends to deviate somewhat from the intended line of cut and also leaves rough, jagged or otherwise irregular severed edges, as well as edges that are non-perpendicular relative to the major surfaces of the sheet. The method and apparatus disclosed in U.S. patent application Ser. No. 767,051, filed Feb. 9, 1977, now U.S. Pat. No. 4,113,162, and assigned to the same assignee as the present invention, provided a solution to these problems by utilizing an electrical resistance wire element maintained in continuous, conductive contact against a surface of the glass to be cut for concentrating or localizing heat along the intended line of cut in conjunction with a mechanical assist for spreading the severed glass portions apart as the thermally induced running cut progressed along the sheet. The mechanical assist prevented dissipation of a significant portion of the thermally induced energy generated in the resistance wire otherwise required to move the severed portions apart and thereby reserved substantially all such thermal energy for use in effecting a true linear cut. However, just prior to reaching the remote or trailing end of the sheet, say within about one inch of the trailing edge of a sheet having a linear dimension on the order of 140-200 inches for example, the running cut frequently tends to curve or flare away from the intended line of cut. This problem of flaring is not peculiar to the above-described process, but is a common problem associated with virtually all known thermal cutting techniques. Generally, such flares can be condoned because the lengths of the severed glass portions are normally trimmed to size along a transverse line inwardly of where the flare starts deviating from the intended line of cut. Nevertheless, it is often desirable to form a true linear cut from starting edge to trailing edge of the blank sheet, and the present invention contemplates avoiding entirely any such deviation or flare at the trailing or exit end of a running cut.